This invention relates to systems for storing nuclear waste material and, more particularly, to apparatus for venting nuclear waste storage containers in a manner that allows gases generated by the stored waste material to escape, while simultaneously minimizing the intrusion of water.
One of the pressing problems currently facing society is the storage and disposal of nuclear waste. Given the magnitude and prolonged duration of the dangers inherent in storing nuclear waste, storage systems must satisfy exacting criteria over long periods of time. Thus, nuclear waste is generally stored in an impervious system specially designed for the application. A typical constraint on the design of such a system is that the waste must be contained, without leakage, for a period of 300 years. Development of a suitable storage system is further complicated by the variety of potential storage locations employed. For example, is frequently stored at the generation site initially. During this time, the storage container is accessible to personnel working at the site, making it susceptible to tampering or accidental damage. The container eventually may be buried at an underground site selected for its geological stability. Burial storage minimizes the likelihood of human interference with the stored waste. In most cases, clay, sand, rock, or salt burial sites are selected to provide a relatively dry storage environment for the container and to minimize the possibility of groundwater contamination. From the preceding discussion, it can be seen that successful storage of nuclear waste requires the system to be resistant to the effects of radiation, erosion, vibration, biodegradation, thermal cycling, burial loading forces, impact forces sustained by the container, and chemical action of the waste and environment on the container.
As noted, the specific problem of nuclear waste storage addressed by this invention is the venting of gas generated within the container. Should these gases cause the internal pressure of the container to become too great, the container structure could become overpressurized, allowing the stored waste to contaminate the environment. Three sources of gas generation within the container must be considered in order to realize a satisfactory venting system. First, the container material itself may generate gas when exposed to the radiation of its contents. Second, ion-exchange resins, which are used to reduce the radioactivity of fluids in nuclear power systems, may undergo radiolytic gas generation when stored in the container. Third, gas may be generated by the biodegradation of organic waste stored in the container (e.g., contaminated grease, solvents, oils, or organic materials attached to the ion-exchange resins). The rate at which gas is generated depends, among other things, on the total radiation dose exposure of the container and contents, the container and ion-exchange resin materials, the amount of organic waste present in the stored material, and the amount of oxygen within the container.
From the preceding discussion, it is clear that a precise determination of the amount of gas will be generated within the container would be difficult at best. Thus, given the need to ensure the structural integrity of the storage container under any set of conditions, a means for venting the interior of the container to the environment must be provided. In that manner, pressure differences between the interior of the container and the environment will be minimized, preventing the container from becoming overpressurized.
It is extremely doubtful that conventional venting devices can meet the design constraints for venting nuclear waste storage containers. For example, the natural venting characteristics of high-density polyethylene, as a container material, are generally incapable of producing the degree of venting required. Small check valves have good water restriction characteristics, but uncertainty exists as to their operation and ability to reseal over the 300-year design life of the container. Filters made of a porous metallic material would appear to have a number of drawbacks. First, their water restriction characteristics appear to be insufficient for nuclear waste storage container applications. Second, the material has a tendency to become wetted and trap water, greatly increasing the pressure required to pass gases through the material. Finally, the use of a metallic material can establish a galvanic couple between the container and the filter and lead to corrosive failure. Activated charcoal filters, while noncorrosive, resistant to gamma radiation, and readily available, generally have a low resistance to the ingress of water.